I am not sure what has been done inside the engine, but it has definitely been jetted to run racing fuel. I am not sure what Mitch Payton does to his bikes to get them ready for racing. See ya at the races!
Here is the listing that the guy posted:
The bike is a 2003 KX125. It was purchased directly from Pro Circuit in early 2004. The bike was built as a race bike for Matt Walker. I do not know if it was ever raced. I have the MSO and bill of sale from Pro Circuit. I have put less than 10 hours on the bike since I purchased it. There are no stories, nothing fishy, and this isn't some cheesy craigslist or ebay dealio. Located in Columbus, OH.
The details:
2003 KX125
welded, gussetted, and powder coated frame
welded and polished subframe
welded and polished swing arm
welded radiators
works Kawi folding tip rear brake pedal
PC engine w/ carb mods (the bike has only had VP MR2 or U2 run through it- must use race fuel)
PC upper and lower triple clamps
PC axle blocks
PC linkage arms
PC carb canister
PC footpegs
PC clutch perch
PC brake snake
PC throttle tube
PC cable guide
PC works pipe and silencer
PC KYB (NOT SHOWA) "Works Kit" suspension for 190lb int. rider
Litespeed carbon clutch cover
Litespeed carbon ignition cover
Litespeed carbon skid plate
Litespeed carbon chain guide
Litespeed carbon front brake line protector
Renthal bars
Renthal sprockets
Excel rims (black with HD spokes)
Watts brake banjo bolts
Does include some spare bits including: some stock parts I was sent in a big zip lock bag, graphics, #30 backgrounds, a signed #30 Matt Walker jersey, and a PC stand.